The objective of the proposed research is to obtain, through the experimental studies and mathematical analysis, a relatively simple description (i.e., model) of heart muscle performances. Our motivation is to find at the myocardial level the basis for our earliest findings on the ventricular contraction that the end-systolic ventricular volume was related to end-systolic pressure in a surprisingly simple manner, regardless of the preloaded end-diastolic volume or afterloaded end-systolic pressure. Therefore, our analysis of muscle mechanics will eventually be directed toward the relation of end-systolic muscle length to end-systolic muscle force under various preload lengths and various modes of contraction. However, to better understand the detailed pathways to this end-systolic force-length relation, instantaneous force-length relation will be studied first in an excised papillary muscle but later in an in vivo muscle preparation. A new loading and measurement system, which is working now, will be further expanded for the in vivo papillary muscle study. The analysis of force-length relation will be extended beyond the conventional analysis of peak force-length curve and force-velocity relation, to F(t)-L(t) diagrams and two first-order phase plane diagrams (i.e., L(t)-L(t) relation and F(t)-F(t) relation). F(t)-L(t) relation will also be explored. The in vivo preparation is less prone to the artifactual series elastance at the cut muscle end and the data will be more directly transferable to the ventricle.